An ability to quickly identify and quantify one or more analytes in a solution is desirable in many areas, including medical diagnostics, petroleum exploration, environmental health monitoring, and drug testing. Unfortunately, many conventional analysis systems and methods are time-intensive and can be quite complicated. In addition, many conventional analytical approaches require the use of consumable reagents or test strips, which require calibration for each use, are subject to degradation over time, often provide only a qualitative result, and can require coding.
Infrared spectroscopy represents an optical chemical analysis method that overcomes many of these drawbacks. Infrared spectroscopy interrogates a sample using an optical signal having a relatively broad wavelength range. Infrared light (electromagnetic radiation having a wavelength within the range of approximately 740 nanometers to approximately 300 microns) is typically transmitted through the sample such that each chemical constituent in the sample imparts spectral information on the outgoing optical signal. This spectral information manifests as intensity peaks at specific wavelength locations in a spectral plot of the output signal, wherein the positions, magnitudes, and inflections of these peaks (i.e., the “spectral fingerprint”) are indicative of the constituent chemicals in the sample.
Initially developed for use in outer space exploration, spectral fingerprinting based on spectroscopy (infrared- and/or visible-light spectroscopy) has been used to measure Doppler shifts caused by radial velocity changes of distant suns in the search for exo-planets potentially orbiting around them. In order to effectively measure such small effects, however, a spectrometer requires careful calibration and an absolute wavelength reference. In space applications, iodine is often used for these purposes. Iodine is an attractive reference because a temperature-controlled iodine vapor cell is spectrally rich over a useful wavelength range. Specifically, iodine has sixty-seven precise and non-variant spectral features over the wavelength range from 389.5 nanometers (nm) to 681.5 nm. An iodine vapor cell is added to the optical path of the interferometer so that the light from the distant sun can pass through it. The spectral features in the light from the sun are then verniered against the iodine spectral features. The Doppler shift of the sun's spectra, therefore, can be precisely determined relative to the absolute locations of the spectral features of the iodine.
The medical industry has embraced infrared spectroscopy for some analytical applications, such as blood analysis, blood flow kinetics, brain scanning, and the like. Unfortunately, in many such applications, the use a separate calibration chemical in the analysis of a chemical mixture is highly undesirable. Often, the analytes being analyzed exist in a background solvent (typically water) at extremely low concentrations. As a result, the spectral characteristics of the background solvent swamp the spectral information of the targeted analytes making them difficult to identify and/or quantify. In addition, in many cases, the background solvent has a high absorption coefficient in the wavelength range where much of the analyte-specific spectral information is located. The addition of more spectral information by using a calibration chemical would often serve only to further confound the analysis of the sample.
Perhaps the most common medical application for which infrared spectrometry is attractive is blood analysis. Unfortunately, infrared spectroscopy of blood chemistry is complicated by the fact that water makes up approximately 80% of blood and the analytes targeted for identification have a concentration level in the parts-per-million range (or lower). Still further, water exhibits a significant absorption window over the range of wavelengths in which most analytes exhibit their strongest characteristic spectral information. As a result, identifying and quantifying an analyte, such as glucose, in blood is complicated by the fact that the spectral signature of the water in the blood masks the spectral information of the analytes.
Currently, many conventional medical analysis systems require collection of blood so that it can be held in a container of known thickness during analysis. This enables the estimation of the concentration of the analyte that is based on the known path length of the infrared light through the sample. The need to draw blood increases patient discomfort, however. It also represents a potential health risk to the caregiver. Ideally, blood analysis would be performed non-invasively by transmitting the infrared radiation through a thin-tissue region of the body, such as the ear lobe or webbing between the fingers. Unfortunately, it is extremely difficult to quantify the measured analytes in the blood in such systems due to the fact that the precise path length of the light through the tissue is indeterminate.
An ability to quantify one or more analytes in a background solution without the use of an external wavelength reference, with the potential for non-invasively measure bodily fluids, would represent a significant advance of the state-of-the-art.